Every day large numbers of syringes are utilized in dispensing fluids. Still the primary device utilized for these injectors is a small cylindrical container or tube, and a plunger piston manually moved within the tube to eject liquid through an outlet at one end of the tube that is connected to a cannula or tubular needle. Some of such syringes are reutilized with proper sterilization in between uses.
A variation of the foregoing, which is in widespread use, is to position a sealed cartridge into a syringe-like holder, and a piston rod or actuator carried by the holder depresses a piston in the cartridge to eject fluid out of a needle attached to the cartridge. The tube and the needle are conveniently disposable; however, it is somewhat cumbersome to remove the cartridges and there is a risk of the operator being stuck with the needle during the removal process.
Considerable efforts have been expended to improve the foregoing apparatus by the use of disposable, plungerless syringes. A search for plungerless syringes has uncovered the following U.S. Pat. No. 2,768,623-Marchand; U.S. Pat. No. 2,911,972-Elinger; U.S. Pat. No. 3,340,869-Bane; U.S. Pat. No. 3,557,788-Swartz; U.S. Pat. No. 3,712,295-Kline; U.S. Pat. No. 4,018,222-McAleer et al.; U.S. Pat. No. 4,130,117-Van Eck; U.S. Pat. No. 4,168,032-Sneider; U.S. Pat. No. 4,282,986-af Ekenstam et al.; U.S. Pat. No. 4,411,656-Cornett, III; U.S. Pat. No. 4,548,601-Lary; and U.S. Pat. No. 4,753,638-Peters.
In general, these patents disclose a variety of small, plastic containers or ampoules which can be squeezed to eject the container contents through an outlet adapted to be connected to a needle to be inserted into a patient. Although these devices have been available for some time, they do not seem to be widely used. One reason for this may be that such plungerless syringes are deemed to present a possible safety hazard to the patient because of the possibility of air or gas being injected into the patient.
During any process of filling a plungerless syringe with liquid, there is inevitably some air or other gas which remains trapped in the container. Likewise there is usually gas within a plunger-type syringe. With a syringe having a plunger, the common technique of removing the gas before injecting liquid into the container is to point the needle of the syringe upwardly causing the gas to rise to the upper end adjacent the outlet. The syringe piston is then depressed sufficiently to force the air out of the syringe. The piston will remain in its partially depressed position so that it is easy for the operator to proceed with injecting the liquid into the patient or infusion device without concern for air being injected.
This procedure cannot be used satisfactorily with the plungerless syringe. While a plungerless syringe can also be oriented with the needle extending upwardly causing the trapped air to rise upwardly, it is very difficult to then carefully move the syringe to a horizontal position and to insert the needle into the patient while holding the plungerless syringe in a partially collapsed position. If the container is squeezed an additional amount during these steps, liquid is, of course, ejected and thus wasted. More importantly, the quantity of fluid being injected into the patient may be unknown and inaccurate. If the collapsing force on the container is relaxed, the container can return to its uncollapsed state, thus drawing contaminated ambient air back into the container through the needle. While the quantity of air initially in a plungerless container is relatively small, and would probably not cause danger to the patient, there is nevertheless a risk involved and hence it is desirable that the air injection risk be eliminated.
The Bane patent, referred to above, discloses a collapsible, bellows-like ampoule wherein, before the needle is inserted, one section of the bellows is collapsed to expel air. Presumably, this is done holding the needle upwardly. A button-like plug holds the section collapsed. While this approach would seem to be effective to remove air, the device is not seen in the market. Possibly the construction may be somewhat expensive.
The above-referenced Lary patent discloses an air vent in a double-walled syringe type container, and refers to ejecting air between a portion of the containers rather than from within the container containing liquid. It does however, prevent air intake into the inner container.
Thus a need exists for an inexpensive syringe that is practically disposable and that eliminates the air injection problem in an improved manner.